Much attention has been given recently to flying spot scanners which impart the information content of a modulated laser beam to a scanned light sensitive medium. Conventional flying spot scanners employing a rotating multi-faceted polygon generally utilize one of two types of illumination procedures. In the first type, the laser beam is adjusted to have a smaller cross-sectional area than the cross-sectional area of the facets of the polygon. With such an arrangement (termed underfilled), all of the laser power is available for use during the scanning operation; however, a facet changeover problem exists because the beam cannot be effectively used during the time that a facet intersection is passing through the laser beam.
In the second type of scanner, the laser beam is made larger than the individual facets of the polygon and, in fact, is spread so as to cover at least two or three adjacent facets simultaneously. With this construction (termed overfilled), there is no facet changeover problem and a high duty cycle is obtainable. However, by spreading the laser beam to fill several facets, a considerable fraction of the potential laser power is not available for use, and this is unacceptable in many applications.
The disadvantages of the underfilled and overfilled flying spot scanners has led to the concept of facet tracking in which an appropriate shifting structure is used for causing the laser beam to follow the active deflecting facet as it moves such that the laser beam remains fully on the active deflecting facet for a period of time at least equal to the duration of the scan across the light sensitive medium. An exemplary facet tracking system of the prior art is disclosed in U.S. Pat. No. 3,910,657 in which a shearing means, associated with each of the deflector facets and positioned in the path of the laser beam in advance of the deflector, shears the laser beam to positions parallel to itself by amounts insufficient to maintain it fully on the deflector face for a substantial period of time. In one configuration of the aforementioned patent, the shearing means is comprised of a plurality of glass plates corresponding in number to the number of facets of the deflector and rotatable with the deflector to vary the amount of shear as the deflector rotates. The exemplary facet tracker is inherently mechanically unstable due to its use of multiple rotating members and hence not desirable for high speed, high resolution scanning. From the foregoing it is apparent that less complex facet tracking systems are desirable.